1. Field of the Invention
The present invention relates to expandable platforms for deploying sensors in a medium; and, in particular, to expandable platforms that use material memory to expand arms that support cables with connected sensors.
2. Description of the Related Art
There are a large number of circumstances in which it is desirable to deploy a group of sensors in a medium according to some predetermined pattern. The predetermined pattern may enclose a one-dimensional linear segment, a two-dimensional area or a three-dimensional volume. Often the sensors are deployed for a limited period of time in remote locations. To efficiently transport the groups of sensors to the remote locations, the groups of sensors are compacted into relatively small packages for transport. At one of the remote locations, a package is launched; and the sensors unfold into the predetermined pattern for deployment. In many cases, the sensors are attached to an expandable platform that is compacted to fit in the transport package, and that expands after launch to an extended, deployed state.
One class of such circumstances involve deploying hydrophones for a limited period of time in natural bodies of water to detect direction or range or both to one or more sources of sound. Recently, volumetric hydrophone arrays have been developed that utilize a web of cables with integrated sensors attached to an expanding platform for delivery by aircraft. The volumetric array packages are designed to fit within conventional airborne package launching systems. A common launching system for large aircraft handles cylindrical packages with a diameter of about five (5) inches (0.12 meters, m) and with a length of about 36 inches (0.9 m). A package that fits in this common launcher is called herein a xe2x80x9cstandard-sized package.xe2x80x9d After allowing space for other components, such as a parachute, surface float, electronics and batteries, the longest allowed platform length within the standard sized package is about 15 inches (0.38 m).
An expanding platform used in packages for the large aircraft is based on multiple telescoping arms that are collapsed for launch and that extend after launch. After launch, the telescoping sections of each arm are extruded from the package as the package falls through the water under its own weight and momentum, with tension provided by a float that rises to the surface of the water. The telescoping arms are connected at their bases by hinges. After the telescoping sections of each arm has been extruded, the base stops falling, held at depth by a tether to the float. The tips of the arms fall under the influence of gravity and the hinges at their bases, until the arms are fully deployed like spokes on a wheel. In certain designs, either the hinges or cables attached to the tips prevent the arms from forming a flat wheel; and, instead, the hinges or cables cause the arms to lie on the surface of a cone, a surface that usually makes a small angle relative to the horizontal. Some such designs arrange the arms so that the cables with the sensors lie in an essentially horizontal plane.
A web of cables connected to the arms often include radial segments that connect the tip of an arm to a point near or vertically displaced a small distance from the base of the arm, and cord segments that connect points near the tips of two different arms. Hydrophones prearranged along the cables form a volumetric array when the platform is deployed.
While suitable for many purposes, the conventional expanding platform suffers several deficiencies for deployment in natural bodies of water. The telescoping joints are points of failure, the maximum length of extension is inadequate for some purposes, and the collapsed telescopes cannot fit in launch systems for small, autonomous aircraft.
In the extended state, each joint where two telescoping sections meet becomes a point of weakness, susceptible to failure when stresses are applied to the arms. Stresses come from vertical motions (xe2x80x9cheavexe2x80x9d) induced by surface wave motions transferred through the surface float, and from horizontal motion variations (xe2x80x9cshearxe2x80x9d) induced by differential currents. Consequently, to survive sea states up to sea state 5 and shears of up to two nautical miles per hour (xe2x80x9cknotsxe2x80x9d) for several days, and still fit in the standard-sized package, arms made of telescoping sections are limited to five sections that extend in sum to no more than about 10 feet (about 3 m).
A maximum ten-foot extension limits the total diameter of the deployed sensor system to a distance, called an xe2x80x9caperture,xe2x80x9d of 20 feet (about 6 m). This aperture is too small for some phenomena of interest. For example, a small amplitude wave in ocean pressure, thermal, salinity, optical or acoustic signals longer than 40 feet (about 12 m) is not readily detected with such a small aperture.
In some applications it is desirable to fit multiple platforms in the standard-sized package. For example, it is desirable to stack two compacted platforms with 20-foot aperture, or to stack more compacted platforms with smaller aperture. Such stacking leaves only about 3 to 7 inches of package length per compacted platform. The telescoping arms cannot fit into such short packages and still provide the apertures desired for some oceanic signals.
A common launching system for small or autonomous aircraft handles cylindrical packages with a diameter of about two inches (0.05 m) and with a length of about 12 inches (about 0.300 m). This is about a quarter the volume of the large aircraft launching systems. The telescoping arms cannot fit in such a small package and still provide the aperture desired for some oceanic signals. Thus the telescoping platform is often not appropriate for autonomous aircraft launch.
For the reasons described above, there is a clear need for an expandable platform that does not suffer the deficiencies of existing expandable platforms.
The past approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not admitted to be prior art merely by inclusion in this section.
According to one aspect of the invention, an expandable platform for deploying sensors in a medium includes a housing and a structural member disposed inside the housing. A releasable restraint is connected to the structural member. Multiple arms are connected to the structural member. Each arm includes a material memory component that has a natural state, corresponding to a minimum energy state, in an extended configuration. The material memory component is energized by compacting the material memory component so that the arm fits within the housing. The material memory component is restrained in an energized state by the restraint, and naturally transforms toward the extended configurations under its own force when the restraint is released. A flexible cable external to the arms is connected to two or more arms, each at a tip portion of the arm. The tip portion is farthest from the structural member when the material memory component is in the extended configuration. Multiple sensors are connected to the cable.
According to another aspect of the invention, a method of fabricating an expandable platform for deploying sensors in a medium includes connecting the sensors to a web of cable segments. Multiple arms are compacted and restrained inside a housing. Each arm includes material having material memory for an extended configuration. The extended configuration has at least one dimension that exceeds any dimension of the housing. The web of cable segments are packed in a package inside the housing and outside the arms. At least one cable segment is attached to a tip portion of each arm. The tip portion is disposed farthest from the housing when the material is in the extended configuration. The platform is prepared for deployment in the medium. This step includes providing for extracting the cable from the package, and freeing each arm of the plurality of arms, after the housing is launched into the medium by a user. Freeing each arm allows the arm to automatically expand outside the housing to the expanded configuration under a motive force supplied by the material memory.
According to another aspect of the invention, an expandable platform for deploying sensors in a medium includes a means for connecting the sensors to a web of cable segments. A means for compacting and restraining multiple arms inside a housing is also included. Each arm includes material having material memory for an extended configuration. At least one dimension of the extended configuration exceeds any dimension of the housing. A means is also included for packing the web of cable segments in a package inside the housing and outside the arms. A means is included for attaching at least one cable segment to a tip portion of each arm. The tip portion is disposed farthest from the housing when the material is in the extended configuration. A means is included for extracting the cable from the package after the housing is launched into the medium by a user. A means is also included for freeing each arm of the plurality of arms after the housing is launched into the medium by a user. The freed arms are allowed to automatically expand outside the housing toward the expanded configuration under a motive force supplied by the material memory.
These aspects allow a user to distribute sensors externally within a medium according to complex, non-linear patterns.